Cutting bevel in mesh surface
Do 3 loop cuts (Ctrl+R) really close to eachother. Then select the one in the middle (Alt+Right Click) and scale it down (S).

Another option would be to separate the mesh by the selected loop. I don't remember if you can do that, but you can always select half the cube and make it a new object (P), then join it back again (Ctrl+J).

Excuse my poor answer but I don't have Blender on this computer to give you some pictures.